Boolean problems, object is left as a hollow mess
You have the right operation i.e Difference, and it is working too, the problem with it is John Cena(you can't see it). Jokes aside!
You only can't see the changes because the object is lying on it. If you want to see the changes before applying the modifier, select the object which you are using for hollowing and go to Object Tab. Under View Port Display Select Display as Wire! Now you can see the hollow.


And also after applying the modifier you can delete that hollow helping object to see the hollow. But its risky, arrange it first with the above method and then you are ready to apply and delete.

Have fun!